This invention relates to plasma generation equipment and techniques, and is particularly directed to automatic tuning of an RF plasma system to match the impedance of a reactive plasma chamber or similar non-linear load to the output of an RF generator or similar RF source. The invention is more particularly concerned with a frequency technique that automatically changes the frequency of an RF generator until it reaches an optimum frequency at which the RF plasma system is tuned.
In a typical RF plasma generator arrangement, a high power RF source produces an RF wave at a given frequency, e.g., 13.56 MHz, and this is furnished along a power conduit to a plasma chamber. The RF power is also typically provided at a fixed, known impedance, e.g., 50 ohms. Because a severe impedance mismatch is typically present between the RF power source and the plasma chamber, some provision must be made for matching the impedance of the plasma chamber to the impedance of the source. In a fixed-frequency RF system, an impedance matching network is interposed between the two. An error detector measures the magnitude error, i.e., the difference between the magnitude of the nominal input impedance (typically 50.OMEGA. and the magnitude of the actual input impedance, and measures the phase error, i.e., the deviation between the phase at the nominal input impedance (typically zero degrees) and the phase at the actual input impedance. The movement of one or more variable impedance devices, used as tuning elements, is governed by the magnitude error signal and the phase error signal. This arrangement can experience long delays in reaching the tuning point, or can experience a "lost" condition where the tuning elements do not seek the tuning point, or may drive the tuning elements away from the tuning point.
Another attractive approach is the frequency tuning technique, in which the frequency of the RF generator is changed until the impedance of the RF plasma chamber matches it as closely as possible. The frequency tuning method has the benefit of not requiring moving parts, and (in theory) achieving a rapid arrival at the optimal tuning point.
With frequency tuning, there is only a single point of control, to wit, frequency. This means that, unlike a mechanically tuned match network, which has at least two variable tuning elements, and can employ three or more, it is possible to achieve perfect tuning with a single load impedance. As a result, frequency tuning is much faster and is mechanically more reliable, as it requires no moving parts. On the other hand, frequency tuning has not been able to achieve the nearly ideal impedance match that a mechanically tuned matching network has achieved.
A typical frequency tuning method operates as follows: The generator is turned on, with its frequency at a starting point within the RF range. The generator supplies forward or applied power to the plasma chamber. A portion of the applied power is reflected back towards the generator. The reflected power is measured, and the magnitude of reflected power is stored in memory. Then the RF frequency is changed in one direction. The reflected power is measured again, and compared with the stored magnitude from the previous measurement. Based on the change in reflected power, the frequency is moved again: if there is a decrease in reflected power, the frequency is moved in the same direction; if there is an increase in reflected power, then the frequency is moved in the opposite direction. This is continued until the lowest possible reflected power is achieved.
A problem arises with this method, because the tuning decision is based solely upon changes in reflected power. The main form of protection for the RF generator involves limiting the RF generator's output power as the load VSWR (voltage standing wave ratio) increases. By limiting the RF output power, the applied power is kept low enough so that the reflected power does not climb above a predetermined threshold. This does not mean that the forward (applied) power is at that same level. When a special limiting condition exists such that the VSWR is high, but not infinite, and the requested power is such that the reflected power is at a maximum level, the existing design for RF tuning cannot detect any change in the reflected power, and cannot effect impedance matching of the generator to the load as the frequency varies. That is, the reflected power is flat across a large part of the frequency range, and the tuning becomes lost.
Another problem that arises is the difficulty in tuning consistently, quickly, and reliably with any requested generator RF output level. In the conventional frequency tuning technique, there is a reverse power threshold that must be changed manually, depending on the requested generator output power range, and on other plasma chamber conditions that affect the VSWR. This creates difficulty in maintaining consistency and reliability from system to system.
In order to avoid the above-mentioned lost tuning condition, the conventional approach has included waiting some predetermined amount of time for the system to tune, and then if tuning has not been achieved, snapping the generator frequency to some predefined point. This works, of course, only if the predefined frequency point is one that will provide a reasonable impedance match for the current plasma chamber conditions, and this may not be the case.
Another previous approach has involved detecting a high reflected power condition and then after some predetermined amount of time quickly scanning the frequency band to look for a better impedance match. However, this solution can be counter-productive, as the algorithm may have been changing frequency in the correct direction when this happens. Thus, this previously proposed solution may result in an oscillatory condition that prevents an acceptable match altogether.
As to the second problem mentioned above, the current approach involves a compromise technique. This compromise is necessary in systems where the best VSWR is relatively high. In such a case, it is required to allow the RF generator to accept a relatively high reflected power as its final tune point. If the requested RF power is also high, this can pose a serious problem that could result in damage to the RF generator. However, if the requested RF power is reduced, there is an increased possibility that the system will settle into a poor impedance match rather than the ideal impedance match.
No one has previously attempted to control the tuning of the RF generator on any basis other than the reflected power alone, and no one has previously appreciated that consideration of the forward or applied power could help resolve the above-mentioned problems.